1st mmap returns *ptr* which aligns to host page size,

    |             size + align               |
    ------------------------------------------
 ptr

input param *align* could be 1M, or 2M, or host page size. After
QEMU_ALIGN_UP, offset will >= 0

2nd mmap use flag MAP_FIXED, then it return ptr+offset, or else fail.
If it success, then we will have something like:

    | offset |          size             |
    --------------------------------------
 ptr      ptr1

*ptr1* is what we really want to return, it equals ptr+offset.

Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>